The present invention belongs to the sector of the control of hydraulic jacks.
Hydraulic jacks are well-known components which convert the hydraulic energy which they receive into mechanical energy which can be considerable. Double-acting jacks consist of a cylinder in which slides a piston dividing the cylinder into two chambers, the piston being connected to an output rod.
When it is intended that the jack should work in one direction in the conventional way, a distribution system is controlled so as to admit fluid under pressure into one of the chambers, while the fluid contained in the other chamber is discharged into a low-pressure reservoir, from which a high-pressure pump draws, to allow the piston to move in the cylinder.
There can be circumstances in which the jack is already subjected to an external force tending to cause the piston to move in one direction and where it is also desirable that the jack should indeed move in this same direction. This occurs, for example, when, after the jack has been actuated, it is desired that it should resume its rest position, corresponding, for example, to the mid-position of the piston in the cylinder, and when the external force to which the jack is subjected is provided by systems for recentering the piston. In general the movement of the piston is obtained once again in the conventional way by the admission of fluid under pressure into one of the chambers and the discharge of the fluid from the other chamber towards the reservoir.
When the external forces are resistant, it will easily be appreciated that the purpose of connecting one of the chambers to the reservoir is to avoid impeding the functioning of the jack. However, in the above-mentioned instances where the jack is already stressed in the direction in which the movement of the piston is to take place, that is to say when the external forces are driving forces, the chamber which is conventionally connected to the reservoir is under pressure precisely as a result of the external stress. Discharging the fluid under pressure to the reservoir therefore obviously constitutes a loss of energy which can be considerable, depending on the particular installation controlled by the jack.